An organic electroluminescence element (OLED) using an organic light-emitting material is promising for use as an inexpensive, large-area, solid-state light-emission-type full-color display element or a writing light source array. Many developments thereon have been made. In general, an organic light-emitting element comprises a light-emitting layer and a pair of opposite electrodes between which the light-emitting layer is sandwiched. When an electric field is applied to between the two electrodes, electrons are injected from the cathode, and holes are injected from the anode, to the layer. In the light-emitting layer, the electrons and the holes are recombined and their energy level returns from the conduction band to the valence band, with emitting the energy as light.
Conventional organic light-emitting elements have problems that the driving voltage thereof is high, and the light-emitting brightness and light-emitting efficiency thereof are low. In recent years, however, many techniques for solving these problems have been reported. An example suggested is an organic light-emitting element wherein an organic thin film is formed by vapor deposition of an organic compound (Applied Physics Letters, Vol. 51, page 913, 1987). This organic light-emitting element has laminated two-layered type structure composed of an electron transport layer and a hole transport layer, and it has a far better light-emitting characteristics than conventional organic light-emitting elements having a single-layered structure. As the hole transporting material of this organic light-emitting element, a low-molecular amine compound is used. As the electron transporting and light-emitting material thereof, an Al complex of 8-quinolinol (Alq) is used. The light emitted therefrom is green. Thereafter, many such organic light-emitting elements wherein an organic thin film is formed by vapor deposition have been reported (see references described in Macromolecular Symposia, Vol. 125, page 1, 1997).
However, such organic light-emitting elements wherein an organic thin film is formed by vapor deposition have a problem that the light-emitting efficiency thereof is far lower than that of inorganic LED elements or fluorescent tubes. In these organic light-emitting elements, a low-molecular compound is formed into a film by a dry method, such as vapor deposition. It is therefore impossible to avoid deterioration in the characteristics thereof due to crystallization of the low-molecular compound. Furthermore, there are problems that costs for production thereof are high, and the production efficiency thereof is low.
To reduce production costs or to apply to flexible large-area elements, such as a backlight and an illumination light source, organic light-emitting elements, wherein a macromolecule light-emitting compound is formed into a film by a wet film-forming method, are developed. Examples of the macromolecular compound include polyparaphenylene vinylene, emitting green light (Nature, Vol. 347, page 539, 1990); poly(3-alkylthiophene), emitting reddish orange light (Japanese Journal of Applied Physics, Vol. 30, page L1938, 1991), and polyalkylfluorene, emitting blue light (Japanese Journal of Applied Physics, Vol. 30, page L1941, 1991). JP-A-2-223188 (“JP-A” means unexamined published Japanese patent application) reports a trial of dispersing a low-molecular compound in a binder resin, and then forming a film from the dispersion by wet coating.
However, the above organic light-emitting elements formed by wet film-forming method also have a problem that the element uses fluorescent luminescence obtained from singlet excitons, in the same way as the above organic light-emitting element formed by dry method, to therefore have only low light-emitting efficiency.
On the other hand, as an inorganic EL element using an inorganic light-emitting material, there are known a thin film-type alternating current driving element, a dispersion-type alternating current driving element, and the like. However, these inorganic light-emitting elements can give only a brightness of several hundred candelas per square meter, although the driving of the inorganic light-emitting element requires a power source supplying several hundreds of volts or hertz. Further, the production of the inorganic EL element is not necessarily easy, since it requires using a gas phase method, such as sputtering, to produce its inorganic light-emitting layer, or dispersing particles having a size of several tens of micrometers into a highly dielectric polymer.
Recently, reported was a charge injection-type, direct-current-driving element using nanoparticles of CdSe, which is an inorganic light-emitting material (H. Mattoussi et al., J. Appl. Phys., Vol. 83(12), 7965 (1998) etc.). However, this material is harmful to both the human body and the environment. Thus, there is a need to develop elements using a safe material.